The study of the effects of cyclic nucleotides on the regulation of microtubule polymerization has been continued. Adenosine, inosine, and guanosine stimulate the phosphorylation of two proteins with molecular weight of approximately 63,000 and 29,000 from 100,000 X g supernatants prepared from porcine brain. The reaction was studied in terms of stability of the phosphate bond, metal ion requirements, nucleoside and nucleotide specificity and mechanism. The phosphorylated proteins are presently being purified.